Exobasidium arescens  

A rarely seen, often inconspicuous, fungal parasite and pathogen of Vaccinium myrtillus (bilberry) characteristic of lower elevations. Visible as small, scarcely thickened, yellow to red leaf spots, 1cm in diameter.

Circumscribed. Leaf-spots small, c. 0.5 cm across, not or only slightly thickened, flat to shallowly cup-shaped, mostly pale (yellowish to buff) but sometimes reddish or even bright red to purple above (in stronger light?), eventually hymenium-clad beneath, after sporulating rapidly drying and turning grey to blackish. Basidia with (3-)4 sterigmata (spore-bearing projections from basidia) up to 6 µm long. Basidiospores 10—14 x 3.0—4.5 µm, plumply musiform (banana-shaped), geniculate (sharply bent) above hilum (connection of sterigmata and basidia), eventually 1—3-septate. Germination by conidia (Sundström). Conidia (4-)5-10 x 0.8—1.5 µm, bacilliform (rod-shaped) to subfusiform (almost spindle-shaped).

Description adapted from Nannfeldt, J. A. (1981). Exobasidium, a taxonomic reassessment applied to the European species. Symbolae botanicae Upsalienses 23(2), 1-72.